The present invention relates to new polypropylene compositions having improved impact strength properties at low temperatures and to a process for preparing same.
As is known, the isotactic polypropylene is endowed with an exceptional combination of excellent properties which render it suitable for a very great number of uses, including appliances at high temperatures; however, it exhibits the drawback of possessing an insufficient impact strength at relatively low temperatures.
Attempts were made to obviate such drawback, without remarkably affecting adversely, however, the other properties of the polymer, either by properly modifying the synthesis process or by blending with rubbers.
The modifications to the synthesis process essentially consist in introducing into the propylene stereoregular homopolymerization process one or more steps of copolymerization of ethylene-propylene mixtures.
The copolymerization conditions of the ethylene-propylene mixtures are selected in such manner as to reduce as much as possible the formation of amorphous ethylene-propylene bipolymer and, on the contrary, to enhance the formation of crystalline ethylene-propylene bipolymer besides, optionally, polyethylene.
This for the reason that the amorphous bipolymer is soluble in hydrocarbon solvents, and in the polymerization processes conducted in the presence of such solvents the formation of the amorphous bipolymer, besides lowering the polypropylene yield, tends to excessively increase the viscosity of the polymerization slurry with ensuing difficulties in the transfer and centrifugation steps of said slurry in order to recover the solid polymer.
In the processes performed in the gas phase the presence of significant amounts of amorphous polymer tends to increase the tackiness of the solid polymeric phase, which results in the fouling of the reactor.
According to the teachings of the prior art, the presence of meaningful amounts of crystalline copolymer is desirable since this results in an improvement of the impact strength properties at low temperatures without remarkably adversely affecting the optical properties (transparency, etc.) and the mechanical properties.
Representative processes and compositions of the art are described in U.S. Pat. Nos. 3,629,368; 3,670,053 and 3,200,173.
The impact strength properties at low temperatures of the isotactic polypropylene can be improved by adding rubbers, in particular ethylene-propylene rubbers, thereto.
According to U.S. Pat. No. 3,627,852 it is necessary, however, to incorporate considerably high amounts of ethylene-propylene rubber in order to achieve a significant improvement.
This involves a drastic worsening of the mechanical properties (flexural rigidity and stability to high temperatures).
Studies accomplished by us allowed to ascertain that the presence of crystalline ethylene-propylene copolymer and optionally of polyethylene in the polypropylene having a high isotacticity index does not contribute to improve the impact strength properties of the polymer; on the contrary, it tends to impair the total balance of the properties.
On the basis of a model studied by us it appears that the impact strength properties of a polypropylene modified with an amorphous ethylene-propylene copolymer substantially depend on the amount and quality of the copolymer.
The function of the copolymer seems to be that of absorbing, at least partially, the impact energy in the area of graft and propagation of the fracture, with consequent improvement of the impact strength of the system.